Modern integrated circuits generally contain several layers of interconnect structures fabricated above a substrate. The substrate may have active devices and/or conductors that are connected by the interconnect structure. As these devices become smaller the need for fine patterning through photolithography has become increasingly more important.
To obtain the fine patterns needed for the current generation of devices, KrF (248 nm) and ArF (193 bm) lasers are being used. The dimension of device will continue to scale down, and may require even shorter wavelengths, such as EUV (13.4 nm). To obtain smaller and finer patterns through the use of shorter wavelength light sources, a new generation of photoresists will be required. The design of the next generation of photoresist may be governed by limitation with EUV tools, such as flare.
When extreme ultraviolet (EUV) steppers are used to expose photoresist to radiation with a wavelength of 13.4 nm, the mid frequency roughness of the optics will cause flare. Flare is produced when the light source is reflected off the reflective optics and passes through the mask to expose the photoresist material, and will reduce the contrast of the aerial image. Small amounts of flare may be able to be corrected for by calculating the amount of flare that will occur and scaling the mask CDs accordingly; however, for higher amounts of flare, alternative strategies are necessary. One strategy is the use of a negative tone resist, especially for poly layers, because the amount of flare is proportional to the amount of light that passes through the mask.
A positive tone photoresist becomes more solulable to a developer solution upon exposure to light, whereas a negative tone photoresist becomes less solulable to a developer solution upon exposure to light. Consequently, when a negative photoresist is used at the poly layer, a dark field mask, instead of a bright field mask, may be used to create a dark field pattern. By using a dark field mask the impact of flare on an underlying layer may be significantly reduced.
Current negative tone photoresists utilize a cross-linking mechanism that makes the exposed portion of the photoresist less solulable to the base developer solution. As cross-linking occurs, the molecular weight of the polymer will decrease. However swelling may occur, since the change in solubility is governed by a change in the molecular weight, and the interactions between the resist and the developer are still very favorable. Swelling of the resist structures will prevent correct pattern transfer and affect the resolution.
Because of these limitations, there is a need for a negative tone photoresist that does not swell during the developing process, thereby, allowing the use of a dark field mask to reduce the effect of flare in patterning.